Kinetically inhibited order in a diamond-lattice antiferromagnet

Abstract

Frustrated magnetic systems exhibit highly degenerate ground states and strong fluctuations, often leading to new physics. An intriguing example of current interest is the antiferromagnet on a diamond lattice, realized physically in A-site spinel materials. This is a prototypical system in three dimensions where frustration arises from competing interactions rather than purely geometric constraints, and theory suggests the possibility of unusual magnetic order at low temperature. Here, we present a comprehensive single-crystal neutron scattering study of CoAl 2 O 4 , a highly frustrated A-site spinel. We observe strong diffuse scattering that peaks at wavevectors associated with Néel ordering. Below the temperature T Ã ¼ 6.5 K, there is a dramatic change in the elastic scattering lineshape accompanied by the emergence of well-defined spin-wave excitations. T Ã had previously been associated with the onset of glassy behavior. Our new results suggest instead that T Ã signifies a first-order phase transition, but with true long-range order inhibited by the kinetic freezing of domain walls. This scenario might be expected to occur widely in frustrated systems containing firstorder phase transitions and is a natural explanation for existing reports of anomalous glassy behavior in other materials. F rustration occurs in spin systems when constraints prevent the formation of a ground state satisfying all of the pairwise interactions. The defining characteristics of frustration are massive ground-state degeneracy and concomitant strong fluctuations. The latter suppress magnetic order and lead to spin-liquid regimes extending to low temperature. There has been great interest in such spin liquids because of the plethora of emergent phenomena (1-3) resulting from an extreme sensitivity to small, often neglected degeneracy-breaking effects. Noteworthy examples include unusual short-range correlations (4, 5), apparent spinglass behavior in the absence of disorder (6-8), collective ring excitations in pyrochlore spinels (9-11), spontaneously reduced dimensionality in rare-earth titanates (12), and topological excitations like magnetic monopoles in spin-ice materials (13-15).The degeneracy-breaking effects can be small terms in the interaction Hamiltonian that enter the problem in a nonperturbative way. Examples include dipolar interactions in the pyrochlores (16) or spin-lattice interactions in multiferroics (17). Interestingly, degeneracy can also be broken in the complete absence of such interaction terms, through a mechanism known as "order-by-disorder" (18). Order-by-disorder refers to the scenario where the entropy associated with thermal or quantum fluctuations lowers the free energy of one ordered state compared to others of equal energy, thereby stabilizing order. Order-by-disorder was first predicted in studies of the diluted Ising model on a rectangular lattice (18). It has since emerged as a central feature in theories of various frustrated antiferromagnets, including the face-centered cubic (FCC) (19,20) as well a...

Spin glass behavior in frustrated quantum spin system CuAl2O4with a possible orbital liquid state

“…Theoretical studies have also shown that with this NNN interaction the normal A-site spinel can exhibit various degenerate magnetic ground states [11]. It should be noted that some A-site normal spinels such as FeAl2O4 and CoAl2O4 are known to display glass-like behavior due to the competition between the NN and NNN interactions for the diamond structure of the A-sublattice [11,12,13].…”

“…Among these TAl2O4 systems, CoAl2O4 is the most frustrated material lying close to a quantum critical point of an antiferromagnetic state [12,16].…”

Spiral spin-liquid and the emergence of a vortex-like state in MnSc2S4

“…Until now, several series of A-site spinels, in which the magnetic A ions form a diamond lattice, have been investigated, including: the cobaltates Co 3 O 4 and CoRh 2 O 4 16 ; the aluminates M Al 2 O 4 with M = Fe, Co, Mn [17][18][19][20] ; and the scandium thiospinels M Sc 2 S 4 with M = Fe, Mn 21 . For the spinels with Fe 2+ at the A-site, the e g orbital angular momentum of Fe 2+ is active, making the pure spin J 1 -J 2 model inadequate 22 .…”

CaMn2Sb2: Spin waves on a frustrated antiferromagnetic honeycomb lattice

“…The exchange term J(Q) describes interactions betweens spins on opposite sublattices and J'(Q) describes interactions between spins on the same sublattice. The absolute value |J(Q)| must be taken because the honeycomb lattice is non-Bravais, like the diamond lattice [39]. We include first neighbor exchange interactions J 1 for spins that are on opposite sublattices, second neighbor interactions J 2 for spins that are on the same sublattice, and exchange interactions between nearest neighbors in different honeycomb layers J c for spins that are on opposite sublattices.…”